As a typical alloy having a shape memory property and a super elastic property, conventionally, a Ti—Ni alloy has been known (for example, see Patent Document 1).
Herein, a shape memory means a phenomenon that a residual strain is eliminated in the case where a metal is heated.
Further, the residual strain means a strain that when a metal with a certain shape firstly given (for example, coil shape) is loaded to stretch beyond its elastic limit leading to a plastic deformation, thereafter, occurs in the metal on unloading.
Additionally, the shape memory is a phenomenon utilizing martensitic transformation-reverse martensitic transformation, for example, it is utilized in a turbine-type heat engine, pipe connection, wire for brassieres and the like.
On the other hand, the superelasticity is referred to a phenomenon that when a metal is loaded in the same manner as described above to stretch beyond its elastic limit leading to a plastic deformation, on unloading from this state, it comes back to the original state (residual strain is almost zero state) while drawing a hysteresis.
The superelasticity generates without increasing a temperature in martensitic transformation-reverse martensitic transformation, so that it is basically the same phenomenon as the shape memory.
In other words, for a metal with a shape memory property, a residual strain is eliminated by heating, whereas for a metal with a super elastic property, a residual strain is eliminated without heating.
A Ti—Ni alloy has a shape memory property and super elastic property in addition to a so-called intermetallic compound composed of at least 2 kinds of metals, and it can be formed in a shape of a plate or line by plastic working.
Further, the martensitic transformation-reverse martensitic transformation of a Ti—Ni alloy is generated at both ends in a specific temperature range near room temperature.
Hence, the Ti—Ni alloy exhibits a shape memory property and a super elastic property in this temperature range.
However, since the Ti—Ni alloy contains a Ni component, it has poor biocompatibility.
Herein, biocompatibility means a degree of metal allergy when skin touches a metal.
Additionally, an excellent biocompatibility means that skin hardly shows a metal allergy.
Therefore, it is difficult to utilize a Ti—Ni alloy as a structural member for an eyeglass contacting facial skin.
Further, as another problem, because a Ti—Ni alloy exhibits the martensitic transformation-reverse martensitic transformation near room temperature, it drastically becomes soft at −5° C. or less.
Namely, a Ti—Ni alloy has a poor low-temperature property.
In this situation, there has been attempted a development of a shape memory and super elastic alloy which has a high-temperature property without containing Ni, which is highly toxic to the human body (see Non-patent Document 1).
Patent Document 1: Japanese Unexamined Patent Publication No. 2002-205164
Non-patent Document 1: “Development of Ti—Sc—Mo shape memory alloy”, The Japan Institute of Metals, lecture abstract p. 144 at springtime meeting, 2003.